Convert BMP to PNG

Convert BMP to PNG without adding lossy compression

BMP is not one single pixel layout. Files can use different DIB headers, bit depths, compression methods, row directions, masks, and palette forms. What they share is age and a habit of consuming far more space than a modern lossless image needs. PNG is the natural replacement when sharp edges, visible detail, or transparency matter and the receiving system accepts a common web and desktop format.

The converter reads the container rather than trusting a .bmp extension. Before conversion it reports dimensions, row orientation, the supported bitmap form, file size, and whether decoded pixels genuinely use transparency. Unsupported compression or ambiguous data produces a named error instead of a guessed image. That matters because a vertically flipped bitmap can still look plausible at a glance while being wrong.

PNG writing is lossless after browser decoding

Writing PNG does not add JPEG-style artifacts. The browser-decoded bitmap pixels enter a lossless output, which makes this route suitable for screenshots, interface assets, diagrams, logos, archived graphics, and files that will be edited again. PNG compression tends to save especially well on flat color and repeated areas. A noisy photograph may shrink less, but the decoded visible content remains the same.

Some BMP variants reserve four bytes per pixel even when the fourth byte is not meaningful alpha. Others really do carry transparent pixels. The page decides from the supported format and decoded result rather than promising transparency merely because an alpha-sized field exists. When transparency is present, PNG can preserve it. No background color is offered on this route because filling it would be destructive and unnecessary.

Browser image decoders and canvas use premultiplied alpha. RGB color hidden behind a fully transparent pixel is therefore not retained, and RGB channels under partial transparency can be rounded. Browsers can also expand some 16-bit BMP channel layouts differently. The preview and receipt describe this browser's decoded pixels; keep the BMP when hidden RGB values or an exact cross-browser 16-bit interpretation are part of the source data you need to preserve.

Choose JPG only for the photographic tradeoff

If the source is a photograph and the smallest practical attachment matters more than exact preservation, the BMP to JPG converter is the sibling job. It adds a quality decision and must flatten transparency because JPEG has no alpha channel. This PNG page deliberately has no quality slider: a control that implies adjustable loss in a lossless format would do nothing.

An optional maximum dimension can reduce an oversized image. It never enlarges a smaller bitmap, and it keeps width and height linked so the picture cannot be stretched. The finished PNG is opened again and its actual dimensions, alpha behavior, and bytes are reported on the receipt before download. PNG stays lossless, so a photograph can still be too heavy for an upload form; in that case you can compress the image to an exact size as a JPEG, which flattens any transparency onto white.

Local conversion, including the verification

Choose a BMP or DIB file or drop it onto the counter. When a clipboard supplies the original bitmap bytes, paste reaches the same path. Parsing, decoding, resizing, PNG writing, and output checks run on this device. Nothing is sent to an image-conversion server and there is no hidden remote fallback. Very large uncompressed bitmaps can require substantial memory, so the page gives advice before work begins but never rejects a file solely because its byte count crosses an arbitrary limit. Nothing about the bitmap is sent anywhere, and the steps to confirm local processing take about a minute with the file you are converting.